A Hearing Assistive Device (HAD), such as a hearing aid, is typically worn at the ear of a user and includes a microphone and a telecoil for receiving audio input. In microphone mode, sound waves are received by the microphone and converted into electrical energy, and the resultant electrical signal is then amplified, processed, and output to the user. In telecoil mode, a telecoil is coupled to an inductive field of a hearing aid compatible device, such as a telephone, to receive audio signals and the signals are amplified, processed, and output to the user.
When using a Wireless Communications Device (WCD), such as a cellular telephone, a HAD user typically places the WCD proximate the HAD in order for the HAD microphone to receive sound emitted from the WCD's speaker. However, the close proximity of the WCD to the HAD may result in unwanted interference as varying RF and magnetic fields associated with the WCD are detected and processed as electrical signals by the HAD. For example, RF interference may result when radio waves emitted by a cellular telephone are detected and demodulated by the HAD circuitry. Wireless devices that employ time division multiplexed modulation schemes may generate interference due to the on/off keying of their modulation envelopes. The pulsing transmissions of such devices may produce interference at the fundamental frequencies associated with the pulse rates, as well as at the associated harmonic frequencies across the audible spectrum.
In addition to RF interference, wireless devices may also generate magnetic interference when placed proximate a HAD. Cellular telephone electronics, such as backlighting, displays, keypads, battery leads and circuit boards may generate pulsed magnetic fields. The resultant magnetic field energy may be coupled with the HAD's wiring and interconnections and result in interference.
Thus, when a WCD is placed proximate a HAD, such as the standard usage position when the WCD's speaker is placed proximate the HAD, unwanted electronic interference often results. Such interference can be diminished by moving the interfering device a distance away from the HAD. Generally speaking, the interference ratio experienced by a hearing aid user as a wireless device is pulled away from the hearing aid is generally represented by a squared relationship. For example, if a wireless device is moved from a first distance x from a hearing aid to a second distance 2x, then the interference generated at the distance 2x is one-quarter that of the interference generated at the distance x. Thus, the interference generated by a WCD may be greatly reduced by moving the WCD a distance from the HAD. Of course, a significant obstacle to moving such devices away from the HAD is the decreased ability of the HAD user to communicate with the WCD, such as the ability to receive audio output from the WCD speaker or to provide speech to the WCD microphone.